Semiconductor and micromechanical devices are typically attached to a substrate. A cover is then attached over the substrate to seal and protect the device inside. The cover can include heat fins or heat pipes or simply be a simple plastic cover that attaches to the outside edge of the substrate. The substrate makes the electrical and mechanical connections to a printed wiring board, a socket, or some other mount. The substrate with the cover attached is called a package. With the increasing complexity of packages and the devices inside new demands are being placed on the substrate and its connections both to the devices inside and to its external connections.
In some cases, a substrate might have different types of electrical connections. The electrical connections will have different electrical requirements for size, density, capacitance, impedance and other characteristics. When the connections on a substrate are created using silkscreen, photolithography, and other similar processes, it can be difficult to create different types of connections using a single set of manufacturing processes.